Liquid crystal display panels (LCDS), particularly color LCD panels, are used for flat screen televisions, projection television systems and camcorder view finders, with many more applications anticipated in the future.
The fabrication of an active matrix liquid crystal display involves several steps. In one step, the front glass panel is prepared. This involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (typically either red, green and blue or yellow, magenta and cyan) color dot or color cell patterns within the spaces outlined by the black matrix. The printed lines which form the black matrix typically are about 15-25 microns wide and about 0.5 to 2 microns thick. The red, green, and blue color cells are typically on the order of about 70-100 microns in width by 200 to 300 microns in length. The color cells are typically printed in films less than about 10 microns thick, and preferably less than 5 microns thick, and must be evenly applied and accurately registered within the pattern formed by the black matrix. The front glass substrate is typically completed by depositing a planarizing layer, a transparent conducting layer, and a polyimide alignment layer over the color filter element. The transparent conducting layer is typically indium tin oxide (ITO), although other materials can also be utilized.
In a second step, a separate (rear) glass panel is used for the formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-off switch for an individual color pixel in the display panel. The third and final step is the assembly of the two panels, including injection of a liquid crystal material between the two panels to form the liquid crystal panel.
One critical step in the manufacture of the color filter is the preparation of the black matrix pattern. The sharpness of edge definition of the black matrix is extremely important. Unlike the colored ink cells, any variation in the black matrix edge, due to printing flow and so forth, is readily discernable when inspecting the final product. The color pixel edge, on the other hand, is typically hidden by the black matrix pattern. Consequently, to a certain extent the black matrix hides variability in the color pixel edge, while there is nothing to hide variability in the black matrix.
The black matrix pattern moreover has a much more stringent registration requirement than the color dot patterns, because the black matrix pattern must be capable of being registered with the transistors that make up the thin film transistor, which is located on the other glass panel. On the other hand, the width of the black matrix pattern provides some leeway in registering the color pixels, because the transition area between individual color dots which make up the color pixels is hidden by the grids which make up the black matrix pattern.
One typical black matrix pattern design consists of elongated black line grids which criss-cross one another to form rectangular spaces within which the color cell pixel inks will be located. Such grid matrices also include smaller black rectangles located along the grid cell edges which correspond to the thin film transistor (TFT) location on the opposite glass panel in the liquid crystal display. Because of the stringent registration requirements, the black matrix is preferably formed so that, for a black matrix pattern having the typical rectangular sub-pixel grid pattern, when looking down at the color filter, only sharp, well defined edges are seen on the black matrix. The cross-sectional shape of the grids which make up such black matrix pattern should also be rectangular with square edges. The corners and intersections of grids in such black matrix patterns should be perpendicular, so that, when looking down at the color filter, only square edges can be seen. Unfortunately, while some printing techniques have successfully been employed to produce the color ink dots which make up the color pixels, the drive to achieve thinner (and thus higher resolution) black matrix lines has pushed the resolution capabilities of conventional printing techniques to their limit. It becomes extremely difficult to maintain the sharp definition using printing techniques as this resolution limit is approached. The biggest problems with such ink printing techniques is that the inks tend to exhibit rounded cross-sectional shapes due to surface tension in the inks, and the edges become irregular.
Consequently, black matrix patterns are typically prepared using photolithographic techniques, even where the remainder of the color filter pattern is produced using printing techniques. Photolithographic techniques involve a large number of production steps, and are much more complex than printing methods. In addition, photolithographic techniques are typically much more expensive than ink printing techniques.
Another critical step in color filter formation is the formation of the red, green and blue color dots (also referred to as color cells) of the color filter. Such color cells preferably should be deposited so that they are as smooth and uniform in thickness as possible. Previous methods used to print color filter patterns have resulted in color patterns having insufficient smoothness. This is largely because the ink depositing methods of the prior art resulted in ink cells which were rounded or triangular in cross section. Consequently, a planarizing layer is commonly applied over the color patterns to alleviate imperfections in coating smoothness or thickness uniformity due to the deposition process. The transparent planarizing layer also serves to protect against ion migration to and from the ITO layer and color pattern layer. The planarizing layer should be deposited to be as smooth and flat as possible.
To facilitate deposition of a flat planarizing layer, it is desirable that the color patterns be smooth, flat and substantially parallel to the undersurface of the glass substrate. Uniform thickness color patterns are desirable for obtaining optimum display contrast and color performance, because if the thickness of the pattern varies, the transmitted light intensity will vary.
One other problem with forming multicolored ink color filter patterns is preventing the different colored inks from mixing with one another. In the past, this problem has been solved by depositing the colors and drying them one at a time.
It would be desirable to provide high quality color filters, having uniform thickness black matrix patterns, as well as ink color filter arrays, which have improved resolution and registration, and which can be obtained easily and at a lower cost than prior art color filter arrays. It would also be desirable to achieve these qualities using a process which takes less steps than current processes.